As information technology (IT) has been rapidly developed and a human's desire for communication has been increased, wireless communication devices including a portable terminal become necessary articles for the present age. However, as these portable devices have been increasingly used, the influence of electromagnetic waves generated from the terminals on the human body becomes an important issue. The influence of electromagnetic waves at a frequency band used by portable terminals on the human body is not yet clearly found, but it has been reported that the electromagnetic waves may cause various diseases such as leukemia, a brain tumor, a headache, a lowering of eyesight, and when they are accumulated in the human body, confusion of brain waves, destruction of men's reproductive function, and the like. Moreover, cases of malfunctions between information communication devices caused by undesired electromagnetic waves are steadily being reported, and steady research on EMI/EMC problems is under way across the world. Thus, many researches for effectively blocking electromagnetic waves to prevent the bad influences of the electromagnetic waves on the human body are being conducted.
An electromagnetic bandgap (EBG) may be implemented by periodically arranging specifically designed unit cell patterns on a typical electric conductor at regular intervals. Since a tangential component of a magnetic field at a particular band on the surface of the EBG becomes zero, the EBG has the characteristic of preventing current from flowing through the surface. Such an EBG may be regarded as a magnetic conductor opposite to a typical electric conductor. The surface of the EBG is a high-impedance surface (HIS) in configuration of a circuit. The frequency response characteristics of the EBG may be checked through a reflection phase which refers to a difference between the phases of an incident wave on the surface of the EBG and a reflected wave from the surface. The reflection phase of the EBG becomes zero at a resonant frequency corresponding to a high impedance surface and varies in a range from −180 degrees to 180 degrees in a frequency band around the resonant frequency. When the structural parameters of the EBG are adjusted, the reflection phase may vary.
In the structure of a typical EBG, a dielectric layer and an array layer for unit cell patterns other than a metal conductive ground plane constitute the typical structure of a frequency selective surface (FSS). FSS is a surface formed by artificially and periodically arranging specific unit cell patterns so as to selectively transmit or reflect desired frequencies. Therefore, an EBG not only completely blocks the progression of electromagnetic waves but also has the above-described unique physical characteristics, by virtue of providing a metal conductive ground plane for the characteristics of filtering of a specific frequency due to the FSS.
Conventional electromagnetic wave absorbers may be variously classified according to a type, material, absorption mechanism, and the like. To date, most electromagnetic wave absorbers have been made of materials formed to have absorption characteristics. Since such electromagnetic wave absorbers are generally developed after much trial and error, it is disadvantageous in that the manufacturing process thereof is complicated and it is highly difficult to adjust an absorption frequency band and absorption characteristics.
To address this problem, a plate-type resonant electromagnetic wave absorber such as a λ/4 wave absorber or a Salisbury screen has been proposed, and is composed of a resistive sheet, a dielectric spacer and a metal conductive ground plane. Therefore, such a plate-type resonant electromagnetic wave absorber is advantageous in that, since its configuration is simplified, its manufacture can be facilitated and absorption performance can be easily adjusted, and in that, when it is constructed in multiple layers, multi-band absorption characteristics can be obtained. However, such a Salisbury screen is disadvantageous in that the thickness of the dielectric spacer from the metal conductive ground plane must be more than at least λ/4.
To implement an electromagnetic wave absorber which is easy to manufacture, makes it easy to adjust an absorption frequency band and absorption characteristics, and can be made thinner, research for a structure of interposing an FSS between the dielectric spacer of the Salisbury screen and the resistive sheet is being carried out.
According to the electromagnetic wave absorber of this structure, the adjustment of thickness and absorption performance is possible owing to the unique electromagnetic properties of the FSS. As a result, an electromagnetic wave absorber formed in this way has a structure formed by adding a resistive coating to the typical structure of the EBG. Furthermore, when the unit cell patterns of the EBG are designed and made of a resistive material on a metal conductor, such a resistive EBG itself may function as a simpler electromagnetic wave absorber. Such an electromagnetic wave absorber may be applied to fields where existing electromagnetic wave absorbers have been applied in order to reduce the multiple reflection of electromagnetic waves, as a simpler structure that is easily manufactured and has low cost. However, this structure has the limitation that only electromagnetic waves in one direction can be absorbed.